1. Field of the Invention
The present invention relates to a band-pass filter for filtering a radio wave having a wavelength range of a high frequency, such as a microwave and a millimeter wave.
2. Description of the Related Art
There have so far been proposed wide varieties of band-pass filters for filtering a radio wave having a wavelength range of a high frequency, such as a microwave and a millimeter wave. This kind of band-pass filter comprises a plurality of resonators, for instance, a wave guide resonator, a cavity resonator or a strip line resonator, being capable of resonating with a desired frequency.
The band-pass filter is utilized for a wide variety of communication equipment which has needed to be miniaturized in recent years. The band-pass filter, therefore, also needs miniaturizing. The use of the strip line resonators can make the band-pass filter to be substantially miniaturized in comparison with the other resonators, i.e., a wave guide resonator or a cavity resonator. For this reason, the band-pass filter including the strip line resonators is useful for the miniaturized communication equipment.
Referring to FIG. 14 of the drawings, there is shown a conventional band-pass filter 1 comprising a plurality of micro strip line resonators represented by the reference numerals 2, 3, 4, 5 and 6 each having a predetermined wavelength for resonating such as a half wavelength .lambda./2 or a quarter wavelength .lambda./4. The resonators 2-6 are arranged on a dielectric substrate 9 in longitudinally parallel relationship and apart from each other at predetermined intervals represented by the reference characters "La, Lb, Lc and Ld" in FIG. 14. The dielectric substrate 9 has a length represented by the reference character "L" as shown in FIG. 14. The length L of the dielectric substrate 9 should be more than the sum of all of intervals La, Lb, Lc, and Ld.
The radio wave signal is inputted to the first resonator 2 through an input terminal 7. The first resonator 2 resonates with the predetermined wavelength. The resonating signal is then transferred from the first resonator 2 to the second resonator 3 by way of the inductive and capacitive coupling. The signal is transferred from the second resonator 3 through the fifth resonator 6 one after another while each of the resonators resonates with its resonating wavelength. The resonating signal is thus outputted from the fifth resonator 6 through an output terminal 8. The band-pass filter 1 can thus obtain the filtered signal having the desired wavelength.
However, a drawback encountered in the conventional band-pass filter of the above-described nature is that the band-pass filter 1 needs a large amount of strip line resonators, so as to obtain a signal having superior characteristics, for instance, a sharp skirt form of a band-edge and a narrow passing band. Furthermore, the band-pass filter 1 needs to extend the space at the interval La, Lb, Lc and Ld in order to reduce the intensity of the coupling between the resonators 2-6. As a result, not only the length L of the dielectric substrate 9 but also the size of the band-pass filter 1 increases.